Dryers of this type are also referred to as through flow dryers for drying sludge of all types and other bulk materials. The bulk material is transported continuously through a dryer housing on a bulk material conveyor. The conveyor inlet and the conveyor outlet of the housing are sealed to reduce the escape of a hot drying gas, usually air. The dryer housing is provided with compression chambers into which the drying air is introduced for passing through the bulk material to be dried. Suction chambers are arranged to collect the air that has passed through the bulk material and through the conveyor that transports the bulk material through the dryer housing. Most of the air from the suction chambers is returned into the circulatory flow of the drying air so that the same hot air is repeatedly used for the drying operation.
Conventional devices of this type generate a substantial quantity of dust which is an undesirable side effect of the drying operation. Such dust has a tendency to be deposited in the above mentioned suction chambers as well as in the compression chambers in all those locations where the air flow is minimal, or where there is a dead air space tending to accummulate dust, whereby the compression and suction chambers are contaminated. Similarly, the air circulating fan or ventilator is exposed to this dust so that frequent cleaning operations are unavoidable. An additional danger exists due to possible dust explosions.
Thus, efforts have been made to clean the repeatedly used drying air, for example, by passing the drying air through a cyclone particle separator, wherein most of the dust is separated before the air is returned into the drying cycle. It is also known to pass the drying air through a stationary filter. Both conventional drying air cleaning operations, either by a cyclone separator or by a stationary filter, have the disadvantage that the efficiency of the separation is unsatisfactory. Besides, a cyclone separator of sufficient capacity for large scale operations is quite involved and correspondingly expensive.